The present invention relates to cylinder head gaskets for internal combustion engines adapted for marine use, and more specifically to a cylinder head gasket for a marine engine which is designed to minimize galvanic corrosion.
Conventional outboard-type marine engines employ ambient water as the coolant. When such engines are used in salt water, the circulation of salt water coolant through the engine has been found to cause accelerated galvanic corrosion of certain engine components. This galvanic corrosion has been particularly troublesome in engines made of non-ferrous metal alloys, such as aluminum alloys. For the sake of brevity, in this application, the term "aluminum" will be understood to refer to aluminum alloys of various compositions.
Salt water induced galvanic corrosion has been found to be especially severe in the cylinder head and engine block of aluminum engines. In these engines, an electrochemical reaction is created when aluminum cylinder heads and engine blocks are joined using a conventional automotive-type cylinder head gasket having a core made of cold rolled steel with a zinc coating, or a stainless steel core. These commonly-used cylinder head gaskets include a metallic core sandwiched between layers of facing material, the latter being used to distribute the clamping load of cylinder head bolts. The type of facing material normally employed has the ability to absorb moisture.
When the engine is operated in salt water, or in some cases polluted fresh water, the water acts as an electrolyte, and an electrochemical cell is induced between the cylinder head and the gasket core, and/or between the engine cylinder block and the gasket core. Even when the engine is not in use, and sea water is no longer circulating, but drains from the engine and motor housing, the still moist facing material prolongs the corrosive electrochemical reaction until the facing material dries out.
Theoretically, the greater the difference in solution potential between the dissimilar contacting metals, the larger the driving force of the electrochemical reaction which causes galvanic corrosion. Since zinc has a higher solution potential than aluminum or steel, the zinc coating of the cylinder head gasket acts as a sacrificial anode and usually dissolves first due to galvanic corrosion. Upon total or at least substantial dissolution of the zinc coating, the cylinder head or engine block then becomes the anode and also begins dissolving. In the case of stainless steel core gaskets not having a zinc coating, the aluminum cylinder head or engine block immediately acts as the anode and begins to dissolve as soon as the electrolyte is introduced.
This condition is a significant deterrent to marine engine longevity, and has been observed to be unusually severe in marine engines having an aluminum cylinder head and an aluminum engine block, when the engines are used in salt water in warm climates. Larger displacement aluminum engines which remain at least partially in the water, even when not in use, are particularly susceptible to galvanic corrosion. Galvanic corrosion has resulted in premature engine block failure, a problem which can only be corrected, short of replacing the engine, by dismantling the engine and replacing the block, an expensive and laborious task.
Despite the magnitude of the galvanic corrosion problem in aluminum marine engines, there has been little incentive for marine engine gasket suppliers to address the problem of galvanic corrosion in nonferrous alloy marine engines. This is due in part to the fewer marine engines produced relative to automotive engines, which forces marine engine manufacturers to commonly obtain cylinder head gaskets from suppliers who provide similar gaskets to automotive engine manufacturers. In addition, aluminum marine engines are but a portion of the total production of marine engines. In view of this situation, most marine engine manufacturers have concentrated their corrosion inhibiting design efforts to changes in the engines themselves.
One attempt which has been made to minimize marine engine galvanic corrosion has been to provide sacrificial water jacket anodes made of zinc or aluminum. These anodes are mounted within the engine cylinder block or cylinder head and are designed to have a greater solution potential than the engine block and cylinder head material. However, when such anodes are placed in the cylinder head, it is difficult to easily determine when the anodes are depleted to the extent that replacement is required. Once the anodes are depleted, if they are not promptly replaced, galvanic corrosion may still begin. Also, the use of sacrificial anodes increases the cost and assembly time of marine engines.
An additional drawback of supplemental sacrificial anodes is that due to their mounting in the water galleries of the engine, adjacent to but not contacting the head gasket, once the engine is no longer operating, these anodes do not prevent galvanic corrosion due to the residual moisture held by the wet facing material.
Another attempt at minimizing galvanic corrosion has been to coat corrosion susceptible engine surfaces with a non-reactive paint or electrochemical barrier coating which is intended to prevent the inducement of the electrochemical reaction. However, the longevity and effectiveness of such coatings in the high temperature, corrosive environments of marine engines has not yet been established.
Accordingly, it is an object of the present invention to provide a gasket for a marine engine which is designed to minimize galvanic corrosion between the gasket and the engine components which contact the gasket.
It is another object of the present invention to provide a gasket for disposition between contacting nonferrous metal surfaces of a marine engine, the gasket having a metallic core which minimizes galvanic corrosion of the gasket as well as the contacting engine surfaces.
It is yet another object of the present invention to provide a cylinder head gasket for a marine engine having a non-ferrous alloy cylinder head and engine block, the gasket having a metallic core which minimizes galvanic corrosion of the gasket as well as the cylinder head and block.